Abstract
The cosmopolitan marine haptophyte alga Emiliania huxleyi accumulates very long-chain (C37-C40) alkyl ketones with two to four trans-type carbon-carbon double bonds (alkenones). These compounds are used as biomarkers of haptophytes and as palaeothermometers for estimating sea-surface temperatures in biogeochemistry. However, the biosynthetic pathway of alkenones in algal cells remains enigmatic, although it is well known that the C37 tri-unsaturated alkenone (K37:3) becomes dominant at low temperatures, either by desaturation of K37:2 or by a separate pathway involving the elongation of tri-unsaturated alkenone precursors. Here, we present experimental evidence regarding K37:3 synthesis. Using the well-known cosmopolitan alkenone producer E. huxleyi, we labelled K37:2 with 13C by incubating cells with 13C-bicarbonate in the light at 25 °C under conditions of little if any K37:3 production. After stabilisation of the 13C-K37:2 level by depleting 13C-bicarbonate from the medium, the temperature was suddenly reduced to 15 °C. The 13C-K37:2 level rapidly decreased, and the 13C-K37:3 level increased, whereas the total 13C-K37 level—namely [K37:2 + K37:3]—remained constant. These 13C-pulse-chase-like experimental results indicate that 13C-K37:2 is converted directly to 13C-K37:3 by a desaturation reaction that is promoted by a cold signal. This clear-cut experimental evidence is indicative of the existence of a cold-signal-triggered desaturation reaction in alkenone biosynthesis.
Highlights
We showed that cerulenin, which inhibits the biosynthesis of C16–C18 fatty acids by inhibiting β-keto-acyl-ACP synthase, suppressed alkenone biosynthesis in E. huxleyi cells[25]
These results suggested that the alkenone biosynthesis pathway was located downstream of the fatty-acid elongation pathway[25]
For the 13C-labelling experiments, E. huxleyi cultures were first incubated with sodium 13C-bicarbonate, which was added as a photosynthetic substrate, under continuous illumination by 20-W fluorescent lamps with an intensity of 100 μmoles/m2/s at 25 °C for 3 days (Fig. 2)
Summary
The 13C-K37:2 level rapidly decreased, and the 13C-K37:3 level increased, whereas the total 13C-K37 level—namely [K37:2 + K37:3]—remained constant These 13C-pulsechase-like experimental results indicate that 13C-K37:2 is converted directly to 13C-K37:3 by a desaturation reaction that is promoted by a cold signal. Emiliania huxleyi (Division, Haptophyta) is a coccolithophore covered with coccoliths made of calcium carbonate crystals It is widely distributed across the polar and equatorial oceans[1,2]. When E. huxleyi cells are grown at the optimum growth temperature (about 25 °C), they produce mainly di-unsaturated alkenones such as K37:2, K38:2, K38:2Et and K39:2. To reconstruct palaeotemperatures by using alkenones extracted from oceanic and lake sediments, many calibrations of the alkenone unsaturation index vs growth temperature have been made experimentally by using various laboratory-cultured coccolithophores grown at different temperatures[10,22,23,24]
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